Tetrafluoroethylene thermoprocessable copolymer microspheres

ABSTRACT

Tetrafluoroethylene (TFE) thermoprocessable copolymer microspheres having a substantially spherical shape for at least 95% by weight, the average size of the microspheres being in the range 25 μm and 2 mm, the bulk density being in the range 0.5 and 1.1 g/cm 3 .

[0001] The present invention relates to tetrafluoroethylenethermoprocessable copolymers under the form of granules having a welldefined morphology and controlled size (hereinafter called“microspheres”), having an improved flowability, to be used in variousapplications, such for example rotomouldidng, powder coating, etc.

[0002] In particular, the invention relates to TFE thermoprocessablecopolymer microspheres having a substantially spherical shape for atleast 95% by weight, the microsphere size being in the range 25 μm and 2mm, the bulk density being in the range 0.5 and 1.1 g/cm³. Thefluoropolymer microspheres of the present invention, having asubstantially spherical shape and a particularly high bulk density, canadvantageously be used in various applications such as for examplepowder coating, rotomoulding, flame spraying and as inert support ingas-chromatography columns.

[0003] It is known that from the polymerization reactor of fluorinatedmonomers a fluorinated polymer latex is obtained which is subsequentlysubjected to the conventional coagulation technique. A coagulum formsunder the form of fine powder, having an average particle size lowerthan 35 μm. The particles have a low bulk density, in the range 0.2-0.5g/cm³, and a substantially irregular, i.e. not spherical, shape such forexample shown in the photo of FIG. 4 (by scanning electronic microscope(SEM)). The coagulum particles have therefore few industrialapplications. For example in applications as the powder coating, anaverage particle size in the range 25-80 μm is required. This means thatonly one portion of the coagulum is obtainable by sieving. However, thesieving of particles having a so small size is difficult since cloggingof the sieves takes place. On the other hand by using the coagulum assuch, a non uniform coating is obtained having different properties.

[0004] Furthermore, due to the very small size, these coagulum particlescannot be used in rotomoulding and rotolining applications. Indeed, forthese applications they must necessarily be extruded and supplied to theusers under the form of milled or unmilled pellets, having sizes from100 μm to 1,000 μm. However, due to their very low bulk density andtheir irregular shape, the coagulum particles flow with difficultyduring the extrusion process. This poor flowability often implies thesetwo drawbacks: 1) a low extruder productivity and 2) block in someextruder section, with consequent bad functioning of the extruder.

[0005] It is furthermore known that in gas-chromatography columns, PTFEas inert support under the form of particles having an average size inthe range 200-500 μm can be used. This porous PTFE support is mainlyformed by particles with an irregular, not spherical, shape, having alow bulk density, in the range 0.3-0.5 g/cm³. Due to the low bulkdensity, it is necessary to carefully use this support, since it easilytends to compress itself and to become a compact solid rather than toremain porous, when it is sbjected to stress forces during the use andduring the gas-chromatography column packing. For this reason, duringthe column packing phase, a cooling under 0° C. is necessary to dispersethe static load and harden this support. Besides, this inert support,because of the non spherical shape of the particles, has an empty spacedegree which considerably changes with the loss of pressure, whereforethe columns result particularly sensible to any variation of the loss ofpressure which can lead to a breaking of the support continuity. Theobtained packing, having a low bulk density, shows furthermoreunsatisfactory mechanical and electric properties negatively affectingthe column efficiency.

[0006] Generally it can be stated that the coagulum particles derivingfrom the conventional coagulation techniques, having a very low bulkdensity, show the drawback to easily disperse in the working environmentdue to their extreme lightness with consequent loss of useful product inthe application phase and problems of environmental pollution for theoperators. Another drawback resides in that they show an irregular or“dendritic” structure, which implies a low flowing capability andtherefore transfer difficulty for clogging problems. This poor flowingcapability limits the effectiveness of their use in all the applicationswhere free-flowing properties are required, for example for the aboveapplications.

[0007] The need was thefore felt to have available for the abovementioned applications of powder coating, rotomoulding, inert supportfor gas-chromatography columns, fluoropolymer microspheres having aregular substantially spheric morphology, such as to confer improvedfree-flowing properties in application phase, and having besides ahigher bulk density, which implies improved mechanical resistance,smaller volume occupied in application phase, lower dispersion in theworking environment.

[0008] An object of the present invention are therefore TFEthermoprocessable copolymer microspheres having a substantiallyspherical shape for at least 95% by weight, the average size of themicrospheres being in the range 25 μm-2 mm, the bulk density being inthe range 0.5-1.1 g/cm³, preferably 0.55-1.0 g/cm³.

[0009] The substantial sphericity of the microspheres is shown in thephoto of FIG. 3 ((SEM)).

[0010] For TFE thermoprocessable copolymers, polymers obtained by TFEpolymerization with one or more monomers containing at least oneethylene type unsaturation are meant.

[0011] Among the TFE comonomer the fluorinated are in particularmentioned:

[0012] C₃-C₈ perfluorolefins, such as hexafluoropropene (HFP);

[0013] C₂-C₈ hydrogenated fluoroolefins, such as vinyl fluoride (VF),vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobutene,perfluoroalkylethylene CH₂═CH—R_(f), wherein R_(f) is a C₁-C₆perfluoroalkyl;

[0014] C₂-C₈ chloro- and/or bromo- and/or iodo-fluoroolefins, such aschlorotrifluoroethylene (CTFE);

[0015] CF₂═CFOR_(f) (per)fluoroalkylvinylethers (PAVE), wherein R_(f) isa C₁-C₆ (per)fluoroalkyl, for example CF₃, C₂F₅, C₃F₇;

[0016] CF₂═CFOX (per)fluoro-oxyalkylvinylethers, wherein X is: a C₁-C₁₂alkyl, or a C₁-C₁₂ oxyalkyl, or a C₁-C₁₂ (per)fluoro-oxyalkyl having oneor more ether groups, for example perfluoro-2-propoxy-propyl;

[0017] fluorodioxoles, preferably perfluorodioxoles;

[0018] non conjugated dienes of the type:

[0019] CF₂═CFOCF₂CF₂CF═CF₂,

[0020] CFX¹═CX²OCX³X⁴OCX²═CX¹F

[0021] wherein X¹ and X², equal to or different from each other, are F,Cl or H; X³ and X⁴, equal to or different from each other, are F or CF₃,which during the polymerization cyclopolymerize.

[0022] Also hydrogenated olefins, preferably in addition to the abovementioned comonomers can be mentioned. Examples of hydrogenated olefinsare ethylene, propylene, butene and isobutene.

[0023] Generally, for the semicrystalline thermoprocessable copolymers,the comonomer amount in the copolymer is between about 0.05 and 18% byweight, preferably 0.5 and 10% by weight, and it depends on the type ofcomonomer.

[0024] The TFE copolymers can also be amorphous-vitrous depending on thecomonomer. They can be obtained for example by using as comonomers thedioxoles or the monomers which cyclize during the polymerization. Inthis case the comonomer amount can be much higher, generally higher than20% by weight.

[0025] Therefore the TFE copolymers of the present invention can besemicrystalline and amorphous-vitrous, provided they arethermoprocessable. The skilled is easily capable to determine, byroutine tests, the comonomer amount to have a thermoprocessable polymer,i.e. thermomouldable.

[0026] Examples of thermoprocessable copolymers are:

[0027] FEP copolymers, i.e. tetrafluoroethylene-hexafluoropropene(TFE/HFP) copolymers, described for example in U.S. Pat. No. 2,946,763;

[0028] FEP copolymers modified with a third monomer, for exampleTFE/HFP/PEVE (perfluoroethylvinylether) copolymers described in EP759,446 and U.S. Pat. No. 5,677,404;

[0029] TFE/HFP/PMVE (perfluoromethylvinylether) copolymers described inU.S. Pat. No. 5,688,885;

[0030] TFE/HFP/PPVE (perfluoropropylvinylether) copolymers described inU.S. Pat. No. 4,029,868.

[0031] In the FEP copolymers the HFP amount is about 5-10% by moles,while the perfluoroalkylvinylether amount in FEPs modified withvinylether is between about 0.2 and 3% by weight.

[0032] Other thermoprocessable copolymers preferred in the presentinvention are:

[0033] TFE/PPVE copolymers commecially called PFA, described in U.S.Pat. No. 3,635,926;

[0034] copolymers TFE/PMVE/fluorinated monomer selected from the abovementioned ones, preferably PPVE, wherein the PMVE amount ranges from 0.5to 13% by weight, the fluorinated monomer amount ranges from 0.5 to 3%by weight. See for example U.S. Pat. No. 5,463,006; the terpolymerTFE/PMVE/PPVE is commercially called MFA.

[0035] copolymers TFE/PMVE/fluorinated dioxole wherein PMVE is in therange 0.5%-13% by weight, the fluorinated dioxole is in the range0.05%-3% by weight. Such copolymers are described in U.S. Pat. No.5,498,682. As fluorinated dioxole,2,2,4-trifluoro-5-trifluoro-methoxy-1,3-dioxole (TTD) is preferablyused.

[0036] The polymerization of the thermoprocessable copolymers of theinvention can be carried out in emulsion according to methods known inthe prior art, preferably according to the processes described in U.S.Pat. No. 5,498,682 and U.S. Pat. No. 5,463,006. Polymerization latexesare obtained from which the microspheres of the invention are obtainedby the process reported hereunder.

[0037] The microspheres of the invention are obtained in an equipmentformed by a coagulation apparatus having a cylindrical shape shown inFIG. 1, where the reference numbers show:

[0038] (1) Jacket for maintaining the temperature in the coagulationapparatus at a desired value;

[0039] (2) Outlet of the coagulated product;

[0040] (3) Coagulant inlet;

[0041] (4) Latex inlet;

[0042] (5) Filter;

[0043] (6) Liquid outlet.

[0044] Preferably in the coagulation apparatus of FIG. 1 the ratiobetween height and internal diameter is in the range 1-3. A series ofstirrers assembled along the axis of the cylindrical body are included.Stirrers can have a type of axial, radial or mixed movement, but theradial movement is preferred. The number of stirrers can range from 1 to4, depending on the ratio between the height of the coagulationapparatus and its internal diameter. When multiple stirrers are used, acombination of different movements can be applied. The stirrer diametercan range between 0.3 and 0.7 with respect to the internal diameter ofthe cylindrical body. It is preferred to have a series of baffles in thecoagulation apparatus and in this case a meaningful clearance betweenthe baffles and the coagulation apparatus wall must be left. The shapeof the coagulation apparatus bottom can be arbitrary, but a conic shapeis preferred since it allows to easily discharge the material from thecoagulation apparatus after coagulation. In the upper part of thecoagulation apparatus a filter (5) is provided so that the fluoropolymermicrospheres are kept inside the coagulation apparatus and do not comeout together with the water during functioning of the coagulationapparatus. Above the filter a vertical pipe is installed, where theliquid height can gradually regulate the pressure in the coagulationapparatus during the coagulation. However such pressure regulationmethod can be replaced by pressure control devices. The height of thevertical pipe h is between 0 and 5 meters.

[0045] Another object of the present invention is a process forobtaining the TFE copolymer microspheres of the present invention. Suchprocess is essential for obtaining the shape and bulk density featuresof the invention. Said process includes the use of the above describedcoagulation apparatus in semi-continuous conditions.

[0046] In the initial conditions the coagulation apparatus is free fromair, filled with water and a coagulant. Among coagulants, acids, basesand salts, for example nitric acid, NaOH, etc., can be mentioned. Allthe coagulants known for the coagulation of the TFE copolymer latexescan be used. Generally it is preferable not to use the coagulants whichgive coloration problems or which modify the polymer properties. Whenthe coagulant is an acid or a base it is used at a concentration between0 and 0.5 moles/litre. When the coagulant is a salt, the coagulantconcentration can even be higher.

[0047] The temperature at which coagulation is carried out is in therange 5′-90° C., preferably 15°-70° C. The pressure is determined by theheight of the above defined vertical pipe of the coagulation apparatus.The mixing rate can range between 5 and 25 rps, preferably 10 and 20rps.

[0048] When temperature, pressure and mixing rate reach the steadystate, the latex coming from the polymerization of the TFE copolymer iscontinuously fed to the semi-continuous stirred coagulation apparatus. Acoagulant is fed continuously separately.

[0049] Water is taken in a continuous way from the upper part of thecoagulation apparatus, where a filter is placed so that thefluoropolymer microspheres do not come out together with the water andthey are therefore kept inside the reactor so as to form microsphereshaving the desired size.

[0050] To obtain the microspheres of the invention the latex and thecoagulant are fed at least in two steps, preferably in three steps.

[0051] In the first step (nucleation step) the polymer concentration ofthe latex ranges from 25 g/litre to 300 g/litre, preferably 50-200g/litre; the latex feeding flow-rate is in the range 5 l/hour-45 l/hour.The time of this first step is lower than 10 minutes.

[0052] The second step (nucleation completion) is optional and consistsin ending the nucleation, for example without any polymer feeding forsome minutes, or feeding only water, or water containing a limitedamount of polymer, for example a polymer flow-rate (product betweenlatex flow-rate and polymer concentration in the latex) corresponding to10% of that fed in the first step.

[0053] The third step (growing step) considers a polymer concentrationof the fed latex between 25 g/litre and up to 300 g/litre, preferablybetween 50 and 200 g/litre; the latex feeding flow-rate is in the range5 l/hour-30 l/hour. The time of this-third step is higher than 15minutes.

[0054] After a total residence time comprised between about 25 minutesand 10 hours, the fluoropolymer microspheres are discharged in adiscontinuous way from the bottom of the coagulation apparatus havingthe size and bulk density of the present invention. Subsequently themicrospheres are subjected to a drying step at a temperature in therange 170°-280° C.

[0055] As said, the microspheres of the TFE copolymers of the inventioncan be used in powder coating applications, wherein dried powders offluoropolymer are sprayed by an electrostatic gun on a metal article inorder to supply anticorrosion coatings to the various equipments used inthe chemical industry.

[0056] The microspheres can besides be used in rotomoulding androtolining applications. The rotomoulding is a rotational mouldingtechnique wherein the fluoropolymer is introduced in a mould having asuitable shape which is heated at high temperatures and maintained underrotation until a fluoropolymer manufactured article having the mouldshape is obtained. Manufactured articles typically obtainable byrotomoulding are tanks, bottles, vessels for silicon wafers. When ametal substratum has to be coated by rotational moulding with aprotective fluoropolymer layer, the fluoropolymer is directly fed insidethe metal substratum which is heated and maintained under rotation untilobtaining a fluoropolymer layer having the desired thickness. Thistechnique is called rotolining and is used for coating pipes, fitting,valves and tanks.

[0057] Another application of the microspheres is the flame sprayingtechnique. It essentially consists in the flowing of the fluoropolymerto be applied as coating through a flame generated by a gas combustion.The fluoropolymer, passing through the flame undergoes a quick heating,so reaches the substratum to be coated in a melting state. By thistechnique it is possible to apply fluoropolymer coatings to ceramic andmetal substrata. Furthermore, when the microspheres pass through aflame, the low molecular weight organic substances, such for examplesurfactants, emulsifiers, etc. evaporate and burn at the flametemperature. Therefore fluoropolymer microspheres are purified by usingthe flame spraying technique thus giving coatings containing a smallamount of contaminants. In this way the obtained coatings do notsubstantially release contaminants during their use. The flame sprayingtechnique is a technique which avoids the use of solvents and thereforeis particularly desired to have coatings with not polluting processes.As combustive gases for the flame, hydrogen, acetylene, methane, etc.,preferably hydrogen, are used. With microspheres, having a narrowdistribution of particle diameters, it is possible to carry out theflame spraying in an optimal way. In fact the fluoropolymer microspheresof the invention show the advantage that when they come into contactwith the flame they melt, but they do not decompose during the residencetime of the polymer in contact with the flame. The residence times mustbe such as to make the polymer to melt to give the coating, but not tocause its decomposition. The fluoropolymer microspheres, having a narrowsize distribution, allow to achieve these results.

[0058] As regards the powder coating, rotomoulding and flame sprayingapplications, the fluoropolymer microspheres have free-flowingproperties and therefore they are suitable in an optimal way to the usein said applications overcoming the mentioned drawbacks of the priorart.

[0059] As regards the application as inert support in gas-chromatographycolumns, the microspheres, having a regular morphology and high bulkdensity, form a polymer packing the empty space degree of which isscarcely depending on the variations of the pressure losses or on cyclicvariations of the temperature. Besides, the microspheres of theinvention show a higher mechanical resistance and a reducedelectrostatic effect with respect to the PTFE particles used in theprior art. At last the packing formed by the microspheres of theinvention shows properties of higher stability and assures a highseparation efficiency of the components in the gas-chromatographycolumn, as shown in the Examples.

[0060] The fluoropolymer microspheres can besides advantageously be usedas support for stationary phases to be used in chromatographicseparation processes in gaseous or liquid phase carried out in adiscontinuous (e.g. preparative columns) or in a continuous way (e.g.annular or simulated mobile bed chromatography).

[0061] Some working Examples of the present invention are reportedhereinafter, the purpose of which is merely illustrative but notlimitative of the scope of the invention itself.

EXAMPLES Example 1

[0062] PFA Microspheres

[0063] A semi-batch coagulation apparatus is used, formed by a reactorhaving an internal diameter of 200 mm and height of 500 mm, equippedwith 3 stirrers of the Rushton turbine type. From the upper part of thecoagulation apparatus the water is continuously taken away and a filter(200 mesh) is placed to keep the fluoropolymer particles inside thecoagulation aparatus. A vertical section of the used semi-batchcoagulation apparatus is shown in FIG. 1.

[0064] The latex of a TFE/PPVE (perfluoropropylvinylether)thermoprocessable copolymer called PFA has been prepared according toExample 1 of U.S. Pat. No. 5,463,006, but by introducing PPVE in thereactor and feeding a gaseous TFE mixture, without usingperfluoromethylvinylether. The introduced PPVE amount is such as to have1.8% by moles of PPVE in the final copolymer. The so obtained latex isfed into said coagulation apparatus free from air and initially filledwith an aqueous solution of HNO₃ (coagulant) at a concentration 0.0631moles/litre. The operating conditions are a mixing rate equal to 15rev/sec and a coagulation temperature equal to 55° C. The latex and, ascoagulant, a HNO₃ solution having a concentration 0.5 moles/litre areseparately and continuously fed to the coagulation apparatus.

[0065] The feeding procedure of the latex and of the coagulant followsthree steps.

[0066] The first step is characterized by the following parameters:

[0067] Feeding flow-rate of latex: 12 litres/hour

[0068] Polymer concentration in the latex: 100 g/litre

[0069] Feeding flow-rate of HNO₃ solution: 1.73 litres/hour

[0070] Length of time: 7 minutes

[0071] The second step is characterized by the following parameters:

[0072] Feeding flow-rate of latex: 6.4 litres/hour

[0073] Polymer concentration in the latex: 50 g/litre

[0074] Feeding flow-rate of HNO₃ solution: 0.92 litres/hour

[0075] Length of time: 13 minutes.

[0076] The third step is characterized by the following parameters:

[0077] Feeding flow-rate of latex: 10 litres/hour

[0078] Polymer concentration in the latex: 100 g/litre

[0079] Feeding flow-rate of HNO₃ solution: 1.44 litres/hour

[0080] Length of time: 75 minutes.

[0081] Subsequently the fluoropolymer microspheres of the invention aredischarged from the bottom of the coagulation apparatus which afterdrying at 265° C. show the following properties:

[0082] Morphology: spherical

[0083] Bulk density: 0.818 g/cm³

[0084] Specific surface B.E.T.: 8 m²/g

[0085] Average microsphere diameter: 230 μm Granulometric distributionof the microspheres: Fraction by weight 0.7 4.0 95.3  (%) Size (μm)45-90  90-180 180-300

Example 2

[0086] MFA Microspheres

[0087] The latex of a copolymer formed by a TFE thermoprocessablepolymer containing 0.9% by weight of PPVE and 6.4% by weight of PMVE,called MFA and obtained by polymerization according to Example 2 of U.S.Pat. No. 5,463,006 is fed to the coagulation apparatus described inExample 1, free from air and initially filled with an aqueous solutionof HNO₃ at a concentration 0.1 moles/litre. A mixing rate equal to 18rev/sec and a coagulation temperature equal to 60° C. are selected asoperating conditions.

[0088] The polymer latex and as coagulant a HNO₃ solution having aconcentration 0.5 moles/litre are separately and continuously fed to thecoagulation apparatus.

[0089] The feeding procedure of the latex and o the coagulant followsthree steps.

[0090] The first step is characterized by the following parameters:

[0091] Feeding flow-rate of latex: 12 litres/hour

[0092] Polymer concentration in the latex: 150 g/litre

[0093] Feeding flow-rate of HNO₃ solution: 3 litres/hour

[0094] Length of time: 10 minutes

[0095] The second step is characterized by the following parameters:

[0096] Feeding flow-rate of latex: 6 litres/hour

[0097] Polymer concentration in the latex: 0.0 g/litre

[0098] Feeding flow-rate of HNO₃ solution: 1.5 litres/hour

[0099] Length of time: 10 minutes.

[0100] The third step is characterized by the following parameters:

[0101] Feeding flow-rate of latex: 10 litres/hour

[0102] Polymer concentration in the latex: 150 g/litre

[0103] Feeding flow-rate of HNO₃ solution: 2.5 litres/hour

[0104] Length of time: 120 minutes.

[0105] Subsequently the fluoropolymer microspheres of the invention aredischarged from the bottom of the coagulation apparatus which afterdrying at 265° C. show the following properties:

[0106] Morphology: spherical

[0107] Bulk density: 0.82 g/cm³

[0108] Specific surface B.E.T.: 10 m²/g

[0109] Average microsphere diameter: 250 μm Granulometric distributionof the microspheres: Fraction by weight 1.7 6.3 42.3  49.7  (%) Size(μm) 45-106 106-180 180-300 300-400

Example 3

[0110] The latex of the TFE/PPVE/PMVE copolymer used in Example 2, isfed to the coagulation apparatus described in Example 1, free from airand initially filled with an aqueous solution of HNO₃ at a concentration0.1 moles/litre. A mixing rate equal to 18 rev/sec and a coagulationtemperature equal to 35° C. are selected as operating conditions.

[0111] The polymer latex and as coagulant a HNO₃ solution having aconcentration 3.54 moles/litre are separately and continuously fed tothe coagulation apparatus.

[0112] The feeding procedure of the latex and of the coagulant followsthree steps.

[0113] The first step is characterized by the following parameters:

[0114] Feeding flow-rate of latex: 40 litres/hour

[0115] Polymer concentration in the latex: 150 g/litre

[0116] Feeding flow-rate of HNO₃ solution: 1.16 litres/hour

[0117] Length of time: 5 minutes

[0118] The second step is characterized by the following parameters:

[0119] Feeding flow-rate of latex: 6.4 litres/hour

[0120] Polymer concentration in the latex: 0.0 g/litre

[0121] Feeding flow-rate of HNO₃ solution: 0.19 litres/hour

[0122] Length of time: 15 minutes.

[0123] The third step is characterized by the following parameters:

[0124] Feeding flow-rate of latex: 20 litres/hour

[0125] Polymer concentration in the latex: 150 g/litre

[0126] Feeding flow-rate of HNO₃ solution: 0.58 litres/hour

[0127] Length of time: 18 minutes.

[0128] Subsequently the fluoropolymer microspheres of the invention aredischarged from the bottom of the coagulation apparatus which afterdrying at 265° C. show the following properties:

[0129] Morphology: spherical

[0130] Bulk density: 0.692 g/cm³

[0131] Average microsphere diameter: 62 μm Granulometric distribution ofthe microspheres: Fraction by wt. (%) 4.5 93.1  2.4 Size (μm) 0-3232-106 106-180

Example 4 (Comparative)

[0132] A batch coagulation apparatus of conventional type, formed by areactor having an internal diameter of 200 mm and a height of 500 mm,equipped with 2 stirrers of the Rushton turbine type, is used.

[0133] 12.5 litres of latex of a copolymer having the compositionindicated in Example 1, are fed to said coagulation apparatus. Thepolymer concentration in the latex is equal to 280 g/litre.

[0134] To the latex subjected to mild stirring, a HNO₃ solution is addeduntil a pH equal to 1 is reached. The mixing rate is increased up to 12rev/sec until a complete particle coagulation.

[0135] The properties of the obtained coagulum particles, after dryingat 265° C. are the following:

[0136] Morphology: non spherical

[0137] Bulk density: 0.45 g/cm³

[0138] Average particle diameter: 30 μm Granulometric distribution ofthe particles: Fraction by wt. (%) 62.8  23.2  14.0  Size (μm) 0-3232-45 45-90

Example 5 (Comparative)

[0139] A batch coagulation apparatus of conventional type, formed by areactor having an internal diameter of 200 mm and a height of 500 mm,equipped with 2 stirrers of the Rushton turbine type, is used.

[0140] 12.5 litres of latex of the copolymer having the compositionindicated in Example 2, are fed to said coagulation apparatus. Thepolymer concentration in the latex is equal to 280 g/litre.

[0141] To the latex subjected to mild stirring, a HNO₃ solution is addeduntil a pH equal to 1 is reached. The mixing rate is increased up to 11rev/sec until a complete particle coagulation.

[0142] The properties of the obtained coagulum particles, after dryingat 265° C. are the following:

[0143] Morphology: spherical

[0144] Bulk density: 0.49 g/cm³

[0145] Average particle diameter: 30 μm Granulometric distribution ofthe particles: Fraction by wt. (%) 61.4  26.6  12.0  Size (μm) 0-3232-45 45-90

[0146] The dendritic morphology is shown in FIG. 4 determined by thescanning electronic microscope (SEM).

Example 6

[0147] Rotomoulding With MFA Microspheres

[0148] The fluoropolymer microspheres of the invention are used formoulding a manufactured article by rotational moulding. The macroscopicparameters which determine the quality of a molded article by rotationalmoulding are:

[0149] absence of bubbles

[0150] absence of clots or agglomerates

[0151] smooth surface

[0152] Microspheres of a copolymer having the composition indicated inExample 2 and having the bulk density and the granulometric distributiondefined in Example 2, have been used to mould an article having a hollowcylinder shape.

[0153] The mould is formed by a carbon steel hollow cylinder having aninternal diameter of 140 mm, a length of 180 mm, a thickness of 3 mm.

[0154] Before rotomoulding it is necessary to pre-heat the metal mouldover the process temperature (for example 400° C.) to remove all theentrapped gases and burn the grease residues. After this treatment themetal surface is cleaned with sand, degreased and made free from oxidesor rust.

[0155] The microspheres obtained in Example 2 in an amount equal to 300g are introduced in the mould through a hole having a 1 cm diameter,placed on one side of the cylinder and having also a venting functionfor the mould. The mould is then introduced in the hot oven and letrotate around a single axis at a rotation rate of 5 rpm. During therotation, the oven temperature is initially maintained at 290° C. for 15minutes and subsequently at 340° C. for 70 minutes.

[0156] Then the oven is slowly cooled, maintaining the mould still underrotation. The mould is then removed from the oven after about 50minutes, when the oven temperature has fallen to about 80° C., toascertain that the material is completely crystallized. A mouldedarticle is obtained having the following properties:

[0157] FINAL ARTICLE THICKNESS: 1.5 mm

[0158] FINAL ARTICLE QUALITY: smooth surface, free from bubbles

[0159] The good free-flowing properties of the fluoropolymermicrospheres of the invention and their spherical morphology allow toobtain a final article of a very good quality.

Example 7

[0160] Rotomoulding With PFA Microspheres of Example 1

[0161] Microspheres of a copolymer having the composition indicated inExample 1 and having the bulk density and granulometric distributiondefined in Example 1, have been used to mould an article having a hollowcylinder shape.

[0162] The mould is formed by a carbon steel hollow cylinder having aninternal diameter of 140 mm, a length of 180 mm, a thickness of 3 mm.

[0163] Before rotomoulding it is necessary to pre-heat the metal mouldover the process temperature (for example 400° C.) to remove all theentrapped gases and burn the grease residues. After this treatment themetal surface is cleaned with sand, degreased and made free from oxidesor rust.

[0164] The microspheres obtained in Example 1 in an amount equal to 300g are introduced in the mould through a hole having a 1 cm diameter,placed on one side of the cylinder and having also a venting functionfor the mould. The mould is then introduced in the hot oven and letrotate around a single axis at a rotation rate of 5 rpm. During therotation, the oven temperature is initially maintained at 310° C. for 15minutes and subsequently at 360° C. for 70 minutes.

[0165] Then the oven is slowly cooled, maintaining the mould still underrotation. The mould is then removed from the oven after about 50minutes, when the oven temperature has fallen to about 80° C., in orderthat the material is completely crystallized. A moulded article isobtained having the following properties:

[0166] FINAL ARTICLE THICKNESS: 1.8 mm

[0167] FINAL ARTICLE QUALITY: smooth surface, free from bubbles

[0168] The good free-flowing properties of the fluoropolymermicrospheres of the invention and their spherical morphology allow toobtain a final article of a very good quality.

Example 8 (Comparative)

[0169] Rotomoulding With Particles of Example 5

[0170] The coagulum particles obtained in Example 5 having themorphological and granulometric properties indicated in Example 5, areused to mould an article by rotomolding.

[0171] The same mould and the same operating conditions of Example 6 areused. A moulded article is obtained having the following properties:

[0172] FINAL-ARTICLE THICKNESS: not determinable due to the surfaceirregularity

[0173] FINAL ARTICLE QUALITY: full of bubbles and clots

[0174] The poor free-flowing properties of the used fluoropolymerparticles and their irregular morphology (not spherical), determine theobtainment of an unusable moulded article for the presence of variousdefects.

Example 9 (Comparative)

[0175] Rotomoulding With PFA Particles of Example 4

[0176] Example 8 has been repeated, but using the coagulum particlesobtained in Example 4 with the morphological and granulometricproperties indicated in Example 4. The final article obtained shows thesame features shown in Example 8.

Example 10

[0177] Powder Coating With MFA Microspheres of Example 3

[0178] The fluoropolymer microspheres of the invention obtained inExample 3, are used in a powder coating application.

[0179] Electrostatic spraying tests on carbon steel plates having100×100 mm sizes, 3 mm and 8 mm thickness, have been carried out.

[0180] The following conditions are used:

[0181] the plates have been pre-heated in an oven at 380° C.;

[0182] the microspheres are hot applied on the plate for an applicationtime of about 40 seconds;

[0183] after spraying the plate has been put again in the oven at 380°C. and left in the oven for 30 minutes.

[0184] The following cases have been examined:

[0185] powder flow-rate respectively of 400 and 150 g/min;

[0186] voltage, respectively of 10 and 30 kV;

[0187] The obtained thicknesses of the fluoropolymer coating range from140 to 200 microns.

[0188] RESULTS: on both plates having a thickness of 3 and 8 mm, both atthe 10 kV and 30 kV voltage a smooth coating and free from bubbles isobtained.

Example 11 (Comparative)

[0189] Powder Coating With Particles of Example 5

[0190] The coagulum particles obtained in Example 5 are used in a powdercoating application.

[0191] Electrostatic spraying tests on carbon steel plates having100×100 mm sizes, 3 mm and 8 mm thickness, have been carried out.

[0192] The following conditions are used:

[0193] the plates have been pre-heated in an oven at 380° C.;

[0194] the particles are hot applied on the plate for an applicationtime of about 40 seconds;

[0195] after spraying the plate has been put again in the oven at 380°C. and left in the oven for 30 minutes.

[0196] The following cases have been examined:

[0197] powder flow-rate respectively of 400 and 150 g/min;

[0198] voltage, respectively of 10 and 30 kV;

[0199] The obtained thicknesses of the fluoropolymer coating range from100 to 280 microns.

[0200] RESULTS: it is noticed that on the plate having a thickness bothof 3 and 8 mm, at a 30 kV voltage a not uniform coating and free frombubbles is obtained.

Example 12

[0201] The microspheres of a copolymer described in Example 2 and havingthe bulk density and granulometric distribution defined in Example 2,are used as inert support in a chromatographic column to separate agaseous mixture formed of F₂ and CF₃CF₂CF₂° F. By sieving, themicrospheres having sizes in the range 250-400 μm are separated for theuse in this application.

[0202] The used column, indicated with (B), has a length of 3 meters, aninternal diameter of 4 mm and uses KEL F® (polychlorotrifluoroethylene)as stationary phase. The separation temperature at which column (B)operates is of 50° C. The graph of FIG. 2 referred to the column (B)shows that, by using as inert support the fluoropolymer microspheres ofthe invention, a very good separation of F₂ from CF₃CF₂CF₂OF isobtained, as it can be deduced from the time distance of the two peaksand from their symmetry.

Example 13 (Comparative)

[0203] PTFE particles commercially called Chromosorb T® having the bulkdensity, effective density and granulometric distribution indicated inTable 1, are used as inert support in a chromatographic column toseparate the same gaseous mixture F₂/CF₃CF₂CF₂° F. of Example 10.

[0204] It is verified that by using the column (A) of Example 10 (length3 m; temperature 50° C.) it is not possible to obtain any separation ofthe two components of the mixture.

[0205] In order to succeed in separating the two components, it isnecessary to use a column having a much higher length and to decreasethe separation temperature to 30° C. The used column, indicated with(A), has a length of 7 meters, an internal diameter of 4 mm and uses KELF® (polychlorotrifluoroethylene) as stationary phase.

[0206] The graph of FIG. 2 referred to the column (A) shows that byusing as inert support Chromosorb T®, although the column has a lengthmore than double with respect to the column (B), there is a lowerseparation of the two components the mixture, as it can be deduced fromthe greater distance of the two peaks and from their poor symmetry.TABLE 1 Example 10 Example 11 (Comp.) Support type MFA microspheresChromosorb T Support shape Spherical Dendritic Support size 250-400250-420 (μm) Bulk density 0.82 0.42 (g/ml) Specific density 2.15 2.39(g/ml) Surface area BET 10 7 (m²/g)

Example 14

[0207] Flame Spraying With Microspheres of a TFE Amorphous Copolymer

[0208] The fluoropolymer microspheres of the invention are used in aflame spraying application. Microspheres of a thermoprocessableamorphous copolymer TFE/TTD(2,2,4-trifluoro-5-trifluoro-methoxy-1,3-dioxole) are used. The molarratio between TFE and TTD is equal to 60/40. Said copolymers aredescribed in U.S. Pat. No. 5,498,682.

[0209] The fluoropolymer microspheres of the invention are fed startingfrom a glass container. They have an average diameter equal to 100μ, thefeeding flow-rate of the microspheres is equal to 0.5 g/min.

[0210] The flame spraying is carried out by a UTP Mini-Spray-Jet® gun.The flame is generated by feeding an oxygen and acetylene mixture. Theoxygen flow drags the fluoropolymer microspheres through the flame. Thedesired temperature in the flame can be obtained by regulating the flowand the molar ratio between oxygen and acetylene.

[0211] Flame spraying tests on an aluminum plate having sizes of 100×100mm, thickness of 5 mm, have been carried out.

[0212] The following conditions are used:

[0213] pressure of oxygen feeding line 3.5 absolute bar;

[0214] pressure of C₂H₂ feeding line=1.5 absolute bar;

[0215] along the flame axis the temperature decreases from 1400° C. to900° C.;

[0216] in the zone comprised between the flame end and the aluminumplate the temperature is equal to 600° C.

[0217] A fluoropolymer film having a thickness of about 200 μm isdeposited on the aluminum plate.

[0218] In Table 2 the values of weight average molecular weight M_(w)measured by GPC at 30° C. in perfluorohexane, of intrinsic viscosity ηmeasured at 30° C. in perfluorohexane, and the amount by weight ofresidual surfactant (ppm) before and after the flame spraying operation,are indicated. TABLE 2 Before the flame After the flame sprayingspraying M_(w) (g/mole) 895,000 326,000 η (cm³/g)    43    30 Residual  1600    20 surfactant (ppm)

[0219] It is noticed that the amount of residual surfactant in thefluoropolymer after the flame spraying is remarkably reduced (two ordersof magnitude). Therefore this Example shows that the flame sprayingtechnique can successfully be used in order to purify the fluoropolymerfrom the presence of possible organic materials having a low molecularweight, such as for example surfactants, emulsifiers, etc.

1. Tetrafluoroethylene (TFE) thermoprocessable copolymer microsphereshaving a substantially spherical shape for at least 95% by weight, theaverage size of the microspheres being in the range 25 μm-2 mm, the bulkdensity being in the range 0.5-1.1 g/cm³, preferably 0.55-1.0 g/cm³. 2.Microspheres according to claim 1, wherein in the TFE thermoprocessablecopolymers the comonomers are, selected from: C₃-C₈ perfluorolefins,such as hexafluoropropene (HFP); C₂-C₈ hydrogenated fluoroolefins, suchas vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene,hexafluoroisobutene, perfluoroalkylethylene CH₂═CH—R_(f), wherein R_(f)is a C₁-C₆ perfluoroalkyl; C₂-C₈ chloro- and/or bromo- and/oriodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE); CF₂═CFOR_(f)(per)fluoroalkylvinylethers (PAVE), wherein R_(f) is a C₁-C₆(per)fluoroalkyl, for example CF₃, C₂F₅, C₃F₇; CF₂═CFOX(per)fluoro-oxyalkylvinylethers, wherein X is: a C₁-C₁₂ alkyl, or aC₁-C₁₂ oxyalkyl, or a C₁-C₁₂ (per)fluoro-oxsyalkyl having one or moreether groups; fluorodioxoles, preferably perfluorodioxoles; nonconjugated dienes of the type: CF₂═CFOCF₂CF₂CF═CF₂,CFX¹═CX²OCX³X⁴OCX²═CX¹F wherein X¹ and X², equal to or different fromeach other, are F, Cl or H; X³ and X⁴, equal to or different from eachother, are F or CF₃, which during the polymerization cyclopolymerize;hydrogenated olefins, preferably in addition to the above mentionedcomonomers.
 3. Microspheres according to claims 1-2, wherein the TFEthermoprocessable copolymers are both semicrystalline andamorphous-vitrous.
 4. Microspheres according to claims 1-3, wherein inthe semicrystalline thermoprocessable copolymers the comonomer amount isbetween about 0.05 and 18% by weight, preferably 0.5 and 10% by weight.5. Microspheres according to claims 1-4, whereien the thermoprocessablecopolymers are selected from: FEP copolymers, i.e.tetrafluoroethylene-hexafluoropropene (TFE/HFP) copolymers; FEPcopolymers modified with a third monomer, for example TFE/HFP/PEVE(perfluoroethylvinylether) copolymers; TFE/HFP/PMVE(perfluoromethylvinylether) copolymers; TFE/HFP/PPVE(perfluoropropylvinylether) copolymers.
 6. Microspheres according toclaim 5, wherein the HFP amount is about 5-10% by moles, while theperfluoroalkylvinylether amount is between about 0.2 and 3% by weight.7. Microspheres according to claims 1-4, wherein the thermoprocessablecopolymers are selected from: TFE/PPVE copolymers; copolymersTFE/PMVE/fluorinated monomer, preferably PPVE, wherein the PMVE amountranges from 0.5 to 13% by weight, the fluorinated monomer amount rangesfrom 0.5 to 3% by weight. copolymers TFE/PMVE/fluorinated dioxolewherein PMVE is in the range 0.5%-13% by weight, the fluorinated dioxoleis in the range 0.05%-3% by weight.
 8. Microspheres according to claim7, wherein the fluorinated dioxole is2,2,4-trifluoro-5-trifluoro-methoxy-1,3-dioxole (TTD).
 9. Equipmentformed by the coagulation apparatus having a cylindrical shape of FIG.1, where the reference numbers show: (1) Jacket for maintaining thetemperature in the coagulation apparatus at a desired value; (2) Outletof the coagulated product; (3) Coagulant inlet; (4) Latex inlet; (5)Filter; (6) Liquid outlet.
 10. A process for obtaining the microspheresaccording to claims 1-8, wherein the equipment of claim 9 is used.
 11. Aprocess according to claim 10, wherein in the initial conditions thecoagulation apparatus is free from air, filled with water and acoagulant selected from acids, bases and salts.
 12. A process accordingto claims 9-11, wherein when the steady state is reached, thepolymerization latex is continuously fed to the semi-continuouscoagulation apparatus; separately a coagulant is fed continuously, whilethe water is taken in a continuous way from the upper part of thecoagulation apparatus by a filter.
 13. A process according to claims9-12, wherein the temperature is in the range 5°-90° C., preferably15°-70° C.; the mixing rate ranges between 5 and 25 rps, preferably 10and 20 rps.
 14. A process according to claims 9-13, wherein the latexand the coagulant are fed at least in two steps, preferably in threesteps.
 15. A process according to claim 14, wherein: in the first stepthe polymer concentration of the latex ranges from 25 g/litre to 300g/litre, preferably 50-200 g/litre; the latex feeding flow-rate is inthe range 5 l/hour-45 l/hour; the time of this step is lower than 10minutes; the second step is optional and consists in ending thenucleation, preferably by feeding a polymer flow-rate corresponding to10% of that fed in the first step; in the third step the polymerconcentration of the fed latex is between 25 g/litre and 300 g/litre,preferably between 50 and 200 g/litre; the latex feeding flow-rate is inthe range 5 l/hour-30 l/hour; the time of this step is higher than 15minutes.
 16. A process according to claims 9-15, wherein after a totalresidence time comprised between about 25 minutes and 10 hours, thefluoropolymer microspheres are discharged in a discontinuous way fromthe bottom of the coagulation apparatus; subsequently the microspheresare subjected to a drying step at a temperature in the range 170°-280°C.
 17. Use of the microspheres according to claims 1-8 in powder coatingand flame spraying applications.
 18. Use of the microspheres accordingto claims 1-8 in rotomoulding and rotolining applications.
 19. Use ofthe microspheres according to claims 1-8 as inert support inchromatographic separation columns in gaseous or liquid phase.